1. Field of the Invention
The present invention relates generally to a laptop-size high-order harmonic generation apparatus using near field enhancement, and, more particularly, to a laptop-size high-order harmonic generation apparatus using near field enhancement which can be fabricated in a small size due to no use of a resonator or optical amplification and which can generate coherent Extreme Ultraviolets (EUVs) or soft X rays while maintaining the repetition rate of incident femtosecond laser pulses.
2. Description of the Related Art
EUVs and X-rays have wavelengths shorter than those of visible rays. EUVs and X-rays can improve measurement resolution based on the limitations of diffraction, which are determined by wavelength, in precision measurements using light. EUVs and X-rays can be used in performing micro measurements, which are pertinent to biotechnology, or nondestructive testing using excellent transmission characteristics by expanding the wavelengths to the X-ray region.
In particular, if a light source having excellent coherence is additionally implemented, various types of applications using the interference and diffraction of light can be achieved. Furthermore, EUVs or X-rays can used in precision spectroscopy, frequency standards measurements, etc. in the EUV or X-ray region because the repetition rate of incident femtosecond laser pulses can be maintained.
One of the methods of generating EUVs or X-rays is a method using a synchrotron. Although the generation of EUVs or X-rays using the synchrotron has advantages in that a great amount of high-quality light can be obtained and a variety of wavelength bands can be obtained at the same time, it has a disadvantage in that it is difficult to construct facilities for generating EUVs or X-rays in the laboratory setting because the facilities are of a huge size and are very expensive.
A high-order harmonic generation method using a femtosecond laser has been recently proposed as a method of solving these problems. Accordingly, it is possible to generate coherent EUVs or soft X-rays using a relatively small laboratory apparatus.
In the generation of high-order harmonics, electrons are ionized by applying a high time-varying electric field to an inert gas, such as Argon (Ar), Neon (Ne) or Xenon (Xe), are moved along trajectories, and are then recombined, so energy corresponding to the sum of the ionization energy and kinetic energy of the electrons is generated as light in the EUV or X-ray band.
In order to generate such high-order harmonics, a minimum light intensity of 1013 w/cm2 is necessary to ionize electrons.
However, a femtosecond laser having a light intensity of about 1011 W/cm2 uses Chirped Pulse Amplification (CPA), which is optical amplification using an external resonator, in order to fulfill the minimum light intensity for generating high-order harmonics.
When one or two femtosecond lasers having a light intensity of about 1011 W/cm2 are connected in series and then amplification is performed, the amount of light sufficient to ionize gas atoms is achieved.
Although the CPA method can achieve a critical value for generating high-order harmonics, it cannot maintain the repetition rate of femtosecond laser pulses and has a KHz-level repetition rate. This cannot achieve a repetition rate high enough to allow the CPA method to be applied to precision spectroscopy. Furthermore, there is a significant problem in that the CPA method causes a complexity in the structure and an increase in the size of an apparatus.